1. Field of the Invention
This invention is directed to an improved permanent antistatic polymeric film, a method of producing such a film, and the antistatic coating composition. In particular, the present invention is directed to an antistatic coating which, when applied to a polymeric film, yields a film having improved permanent antistatic properties.
2. Prior Art
The growth of packaging, reprographic, microfilm and magnetic markets has significantly increased the utilization of plastic film materials in these markets. In particular, biaxially oriented plastic film made from polyethylene, polypropylene, nylon, polyester, polystyrene and polycarbonate, may be employed for these purposes. One of the problems associated with the use of such biaxially oriented films is the tendency of these materials to accumulate an electrostatic charge on the surface as a result of handling or processing of the film, especially at low relative humidity. The nonconductive nature of most polymeric films allows for an accumulation of high electrostatic potential. These electrostatic charges are most pronounced during surface separation of the film, such as winding or unwinding, or further processing the film over rolls and similar equipment. The electrostatic charges attract not only dust and other contaminants, but additionally attract other film, such as the next layer of film on a roll of film, causing film blocking during processing. While slip agents can be added to polymeric films to reduce blocking, they do not reduce or eliminate the electrostatic charges. To overcome this serious problem, antistatic films have been developed. Antistatic films are usually formed by the topical treatment of an antistatic coating onto one or both surfaces of the film. Many of the coatings have successfully reduced the electrostatic properties of the film to satisfactorily low levels. However, the coatings are easily removed (i.e. temporary rather than permanent) from the surface by routine contact (rub-off) or by using organic solvents or water during further processing of the film (dissolve-off).
Various antistatic coatings are known in the prior art. U.S. Pat. Nos. 4,214,035 and 4,302,505 to Heberger, disclose a polymeric film having a latex coating comprising a nitrate salt and a polyacrylate or acrylic terpolymer. While this antistatic coating has suitable slip, the antistatic coating is not permanent and can easily be removed in a hot aqueous bath and can be rubbed off.
Another antistatic coated film is disclosed in U.S. Pat. No. 4,642,263 to Culbertson which discloses an antistatic film having a polymeric binder and a quaternary ammonium salt. A film coated with such an antistatic coating composition has good clarity, good slip properties, and remarkably good antistatic properties, but the antistatic properties can be substantially reduced by submitting the film to a hot aqueous bath.
European Patent Application 0-512,540, published Nov. 11, 1992 discloses antistatic coated film which is not easily removed in a hot aqueous bath. The antistatic coating composition is a copolymer which is comprised of (on a dry weight basis): 1) from 60-85% by weight of a quaternary salt of either alkylamino acrylate or alkylamino acrylamide; 2) from 15-40% by weight of one or more acrylates; and 3) from 0-10% by weight of a suitable crosslinker. In addition, the inclusion of an antistatic compound to the antistatic coating composition described above has been found particularly useful in maintaining antistatic properties under manufacturing conditions. Suitable antistatic compounds for use include any long chain quaternary ammonium compound. However, the resulting film is slightly hazy, thus its end use for layout montage could be improved.
Another problem associated with prior antistatic films is that many of the antistatic coatings cause blocking. Blocking occurs when the antistatic layer on the film has a coefficient of friction sufficiently high that one layer of the film does not easily slip over another, or over rollers, or other equipment. Thus, there is a need to develop an antistatic coating which has the ability to remain bonded with the surface of the film, i.e., not rubbed off, as well as developing an antistatic coated film which has a coefficient of friction sufficiently low so that the film has acceptable slip properties, and developing an antistatic film which has no, or only slight haze.